Coated articles are known in the art for use in window applications such as insulating glass (IG) window units, vehicle windows, and/or the like. It is known that in certain instances, it is desirable to heat treat (e.g., thermally temper, heat bend and/or heat strengthen) such coated articles for purposes of tempering, bending, or the like. Heat treatment (HT) of coated articles typically requires use of temperature(s) of at least 580 degrees C., more preferably of at least about 600 degrees C. and still more preferably of at least 620 degrees C. Such high temperatures (e.g., for 5-10 minutes or more) often cause coatings to break down and/or deteriorate or change in an unpredictable manner. Thus, it is desirable for coatings to be able to withstand such heat treatments (e.g., thermal tempering), if desired, in a predictable manner that does not significantly damage the coating.
In certain situations, designers of coated articles strive for a combination of desirable visible transmission, desirable color, low emissivity (or emittance), and low sheet resistance (Rs). Low-emissivity (low-E) and low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors.
U.S. Pat. No. 7,521,096, incorporated herein by reference, discloses a low-E coating which uses zinc oxide (ZnO) contact layers below the silver-based IR reflecting layers, and above the bottom silver (Ag) based IR reflecting layer uses a NiCrOx contact layer followed by a center tin oxide (SnO2) dielectric layer. While the ZnO contact layers below the silver IR reflecting layers provide good structural properties for the growth of silver, the ZnO has been found to degrade the chemical, environmental and mechanical durability of the coating in certain instances. Moreover, the thick SnO2 dielectric layer has been found to show micro crystallization and stress upon HT which causes rough interfaces between the SnO2, the ZnO and the Ag, which can lead to degradation of durability and affect transmitted color.
U.S. Pat. No. 5,557,462 discloses a low-E coating with a layer stack of SiN/NiCr/Ag/NiCr/SiN/NiCr/Ag/NiCr/SiN. However, the coated article of the '462 patent is designed for a high visible transmission of at least 63%. The '462 patent at column 3, lines 12-15, teaches that visible transmission below 70% (monolithic coated article) and below 63% (IG window unit) are undesirable. Thus, the '462 patent teaches directly away from coated articles with visible transmission lower than 63%. Moreover, as largely explained in U.S. Pat. No. 8,173,263, coated articles of the '462 patent are not heat treatable because upon heat treatment sheet resistance (Rs) goes way up such as from about 3-5 to well over 10, haze tends to set in, and the glass side reflective ΔE* value is undesirable because it is over 5.0.
Accordingly, it would be desirable to provided a coated article that is characterized by one or more of: (i) low visible transmission, (ii) good durability, and (iii) thermal stability upon HT so as to realize a glass side reflective ΔE* value no greater than about 5.0, more preferably no greater than about 4.5.
The term ΔE* (and ΔE) is well understood in the art and is reported, along with various techniques for determining it, in ASTM 2244-93 as well as being reported in Hunter et. al., The Measurement of Appearance, 2nd Ed. Cptr. 9, page 162 et seq. [John Wiley & Sons, 1987]. As used in the art, ΔE* (and ΔE) is a way of adequately expressing the change (or lack thereof) in reflectance and/or transmittance (and thus color appearance, as well) in an article after or due to heat treatment. ΔE may be calculated by the “ab” technique, or by the Hunter technique (designated by employing a subscript “H”). ΔE corresponds to the Hunter Lab L, a, b scale (or Lh, ah, bh). Similarly, ΔE* corresponds to the CIE LAB Scale L*, a*, b*. Both are deemed useful, and equivalent for the purposes of this invention. For example, as reported in Hunter et. al. referenced above, the rectangular coordinate/scale technique (CIE LAB 1976) known as the L*, a*, b* scale may be used, wherein: L* is (CIE 1976) lightness units; a* is (CIE 1976) red-green units; b* is (CIE 1976) yellow-blue units; and the distance ΔE* between L*o a*o b*o and L*1 a*1 b*1 is: ΔE*=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2, where: ΔL*=L*1−L*o; Δa*=a*1−a*o; Δb*=b*1−b*o; where the subscript “o” represents the coating (coated article) before heat treatment and the subscript “1” represents the coating (coated article) after heat treatment; and the numbers employed (e.g., a*, b*, L*) are those calculated by the aforesaid (CIE LAB 1976) L*, a*, b* coordinate technique. When, for example, glass side reflective ΔE* values are measured, then glass side reflective a*, b* and L* values are used. In a similar manner, ΔE may be calculated using the above equation for ΔE*, i.e., ΔE*=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2, by replacing a*, b*, L* with Hunter Lab values ah, bh, Lh. Also within the scope of this invention and the quantification of ΔE* are the equivalent numbers if converted to those calculated by any other technique employing the same concept of ΔE* as defined above.